A RFID tag or electronic barcode is generally used to provide identification or other information about a product to which the tag is attached through a wireless link to a reader system which captures this information and passes it on, typically in digital form, to various database, decision-making, or other electronic tracking systems. This information is gathered wirelessly by the RF transmit and receive components of the reader device which typically broadcasts a carrier frequency which can provide RF power, clock signal, and modulation-encoded commands.
In the case of passive tags, which are generally most interesting for low cost tags as they avoid on-tag power source costs, the carrier frequency signal provides the RF energy to power the chip. Clock signal recovery and synchronization are also important system attributes/functions which are usually derived from the reader→tag RF signals. The clock frequency can define the operating frequency and data communication rates from tag to reader and from reader to tag.
At HF, due to frequency bandwidth concerns imposed by national and international regulations, the clock signal is often derived by the tag circuit by dividing down the carrier frequency. At UHF frequencies and above, clock signals are typically derived from subcarrier frequency modulations on the carrier frequency. This is due to a number of reasons. Around 869 MHz and 915 MHz, bandwidth constraints are less restrictive than at HF frequencies in Europe and the U.S., respectively. This allows for the addition of subcarrier modulation of a sufficient frequency to allow high speed data communication between reader and tag. Also, dividing down the carrier frequency directly requires GHz-speed clocking circuits and their associated energy losses. Instead, a 104-105 Hz sub-carrier signal can be demodulated or modulated with simple, lower loss subcircuits that can be made with thin film transistors (TFTs), diodes, capacitors, inductors and resistors.
Communication from tag to reader generally occurs through impedance modulation. In the HF range and lower, the tag is usually in the near field, inductive-coupling range, significantly less than the free space wavelength of the RF carrier. In this case, there is a direct inductive coupling between the tag, which typically has a resonant inductor-capacitor (LC) loop tuned at or near the carrier frequency, and reader as in the primary and secondary coils of a simple inductor-based AC transformer. Modulation of the resonance characteristics of the LC loop in the tag, typically through a variable resistive load (which can be provided by a transistor), results in a detectable impedance change in the reader front end circuit. The tag circuitry serially reads out data via this modulation signal to the reader.
At UHF frequencies, the reader to tag distance is generally longer, and the carrier wavelength is shorter. Due to this, the RF link between the two falls in the range of electromagnetic wave propagation physics, as is typically the case in radar, AM/FM radio or cellular phone technology. In this case, the tag links to the reader via a reflected backscatter signal. By modulating the impedance of the tag's antenna(e), the amount of power or the phase or frequency of the signal reflected back to the reader can be changed, and a time-varying signal can be encoded with this form of modulation. This modulation can be performed resistively, as with a transistor, or through the use of varactors that modulate the imaginary part of the tag antennae's impedance.
On a more basic level, RFID tag circuitry generally performs some or all of the following functions:                1. Absorption of RF energy from the reader field.        2. Conversion of this RF signal into a DC signal that powers the chip.        3. Demodulation of incoming clock, timing and/or command signals available in the RF signal from the reader.        4. State machine decision making and control logic that acts on incoming or preset instructions.        5. Counter- or register-based reading of data in digital form from a memory array or other source (example: output of a sensor).        6. Storage elements (e.g., memory) that store the ID code or other information that is to be read out to the reader and/or used for security authentication.        7. Modulation of coded data, timing signals or other commands back to the tag antenna(e) for transmission to the tag reader        